The present invention relates to imaging apparatus and methods, and more particularly relates to an imaging apparatus and method capable of ejecting liquid structures, which become single liquid drops, without accompanying satellite drops, before reaching a receiver surface.
Inkjet imaging devices use the controlled ejection of small droplets of liquid, to produce an image. Typically, the liquid is ejected through one or more nozzle orifices, which are produced in a nozzle plate. The pressure pulse, which ejects the liquid drop through a nozzle orifice is typically produced by the application of an electrical drive waveform to an electromechanical transducer, as in a piezoelectric printhead; or to an electrothermal transducer, or resistor, as in a thermal printhead. The present invention concerns electrical drive waveforms particularly designed for printing images requiring accurate and artifact-free deposition of the liquid drops on the receiving medium, as for example in graphic arts printing. Examples of ink or printing liquids used with lithographic printing plates are described in U.S. Pat. No. 6,044,762; however, the invention is not limited to the fluids mentioned only in that patent but applies to other fluids suited for ejection from an inkjet printhead as taught herein which are generally referred to herein as an ink or printing liquid.
In the field of continuous inkjet, in which a continuous pressurized fluid jet is caused to break into drops in synchronization with a vibrating transducer, and imagewise caused to deflect, some prior work in the art has been done on the suppression of unwanted satellite drops. For reference example, Keur et al. in U.S. Pat. No. 3,683,396 discloses a method of nozzle design in which the mechanical resonance frequency of the nozzle is chosen to minimize the occurrence of satellite drops. Togawa et al., in U.S. Pat. No. 4,368,474, discloses a charge detector that detects the presence of satellite drops, and regulates a voltage applied to a vibrating transducer, to suppress the satellites.
In the field of drop-on-demand inkjet, in which a drop of liquid is ejected from a nozzle only upon application of an electrical drive signal to an actuator in communication with the nozzles, some prior work in the art has been done on the suppression of satellite drops. For reference example, Lorenze et al. in U.S. Pat. No. 5,461,406 discloses a method of designing a front face, or nozzle, to eliminate misdirected satellite drops in a thermal inkjet printhead.
However, none of the above references address the problem of suppressing or eliminating satellite drops, using an electrical drive waveform particularly designed for ejection of a particular liquid type. It is accordingly an object of the present invention to provide a method and apparatus for forming such liquid drops without satellites, in order to allow accurate and artifact-free placements of the drops onto a receiving medium.
It has been known to use an inkjet printhead to eject drops of liquid onto the surface of a receiving medium to produce an image, as shown in FIG. 1. However, a problem with the prior art has been that in actual practice, the liquid structure that is actually ejected from the printhead nozzle may consist of a liquid droplet connected to or followed by, a ligament or tail, which in turn may break up into a series of satellite drops. This is illustrated schematically in FIG. 1, and in actual practice, in the stroboscopic photomicrographs of FIG. 2b and FIG. 2c. If a receiver in relative motion to the printhead were placed close to the nozzle plate in a position to receive the ejected drops, as for example at the head position of the droplet-satellite object and FIG. 2c, then a mark on the receiver would be formed in the shape of a large dot followed by a succession of small satellite dots, which is undesirable.
It is, therefore, an object of the present invention to provide a method and apparatus of producing liquid structures, which become single drops of liquid, prior to the time that the liquid drops contact the surface of a moving receiver.
Advantage of such a method is that images free of artifacts such as satellite dots, may be produced. Another advantage of such a method is that images requiring high resolution and accurately produced dot structures, such as graphic arts images, may be produced.
In accordance with a first aspect of the invention there is provided a method of operating an ink jet printhead comprising providing an inkjet orifice of the printhead located within a predetermined spacing of less than 1000 micrometers from a receiver member that is moving relative to the orifice so as to present different portions of the receiver member to the orifice at the predetermined spacing; providing electrical drive signals to the printhead, the electrical drive signals being adapted to enable the printhead to generate a droplet of a printing liquid; and forming a free droplet of the printing liquid substantially free of any satellites between the orifice and the receiver member and depositing the droplet upon the receiver member.
In accordance with a second aspect of the invention there is provided an inkjet printing apparatus comprising a printhead having an inkjet orifice within a predetermined spacing of less than 1000 micrometers from a receiver member that is moving relative to the orifice so as to present different portions of the receiver member to the orifice at the predetermined spacing; and a source of electrical drive signals to the printhead, the electrical drive signals being adapted to enable the printhead to generate a free droplet substantially without presence of any satellites that would otherwise form a mark on the receiver member.